November 29, 1961: The SL-1 reactor vessel being removed from the reactor building, which acted substantially like the containment building used in modern nuclear facilities. The 60-ton Manitowoc Model 3900 crane had a 5.25-inch (13.3 cm) steel shield with a 9-inch (23 cm) thick lead glass window to protect the operator.

The SL-1, or Stationary Low-Power Reactor Number One, was a United States Army experimental nuclear power reactor which underwent a steam explosion and meltdown on January 3, 1961, killing its three operators. The direct cause was the improper withdrawal of the central control rod, responsible for absorbing neutrons in the reactor core. The event is the only known reactor incident in the United States which resulted in immediate fatalities.[1][2] The incident released about 80 curies (3.0 TBq) of iodine-131,[3] which was not considered significant due to its location in the remote high desert of easternIdaho. About 1,100 curies (41 TBq) of fission products were released into the atmosphere.[4]

The ALPR before the accident. The large cylindrical building holds the nuclear reactor embedded in gravel at the bottom, the main operating area or operating floor in the middle, and the condenser fan room near the top. Miscellaneous support and administration buildings surround it.

From 1954 to 1955, the U.S. Army evaluated their need for nuclear reactor plants that would be operable in remote regions of the Arctic. The reactors were to replace diesel generators and boilers that provided electricity and space heating for the Army's radar stations. The Army Reactors Branch formed the guidelines for the project and contracted with Argonne National Laboratory to design, build, and test a prototype reactor plant to be called the Argonne Low Power Reactor (ALPR).

Some of the more important criteria included:

All components able to be transported by air

Use of standard components

Minimal on-site construction

Simplicity and reliability

3-year fuel operating lifetime per core loading

The prototype was constructed at the NRTS site from July 1957 to July 1958. It was operational on October 24, 1958. The 3 MW (thermal) boiling water reactor (BWR) used highly enriched uranium fuel. It operated with natural circulation, using light water as a coolant and moderator. ANL used its experience from the BORAX experiments to design the BWR. The system operated at 300 pounds per square inch (2,100 kPa) using 90 percent U-235 in the fuel plates, made of a uranium-aluminum alloy. The plant was turned over to the U.S. Army in December 1958 after extensive testing, with Combustion Engineering acting as the lead contractor beginning in February 1959. Members of the U.S. Army, called cadre, began training as plant operators.

The majority of the plant equipment was located in a cylindrical steel reactor building 38.5 feet (11.7 m) in diameter and an overall height of 48 feet (15 m) The building was made of plate steel, most of which had a thickness of 1/4 inch (6 mm). Access to the building was provided by ordinary doors. The building was not a pressure-type containment shell as would have been used for reactors located in populated areas. Nevertheless, the building was able to contain most of the radioactive particles released by the eventual explosion.

The core structure was built for a capacity of 59 fuel assemblies, one source assembly, and 9 control rods. The core in use, however, had 40 fuel elements and was controlled by 5 cruciform rods. The control rods were made of 60 mils (1.5 mm) thick cadmium, clad with 80 mils (2.0 mm) of aluminum. They had an overall span of 14 inches (36 cm) and an effective length of 32 inches (81 cm) . The 40 fuel assemblies were composed of 9 fuel plates each. The plates were 120 mils (3.0 mm) thick consisting of 50 mils (1.3 mm) of uranium-aluminium alloy "meat" covered by 35 mils (0.89 mm) of X-8001 aluminum cladding. The meat was 25.8 inches (66 cm) long and 3.5 inches (8.9 cm) wide. The water gap between fuel plates was 310 mils (7.9 mm). The initial loading of the 40 assembly core was highly enriched and contained 31 pounds (14 kg) of uranium-235.

The deliberate choice of a smaller fuel loading made the region near the center more active than it would have been with 59 fuel assemblies. The four outer control rods weren't even used in the smaller core. The effort to minimize the size of the core gave the central rod an abnormally large reactivity worth.

On December 21, 1960, the reactor was shut down for maintenance, calibration of the instruments, installation of auxiliary instruments, and installation of 44 flux wires to monitor the neutron flux levels in the reactor core. The wires were made of aluminum, and contained slugs of aluminum–cobaltalloy.

On January 3, 1961, the reactor was being prepared for restart after a shutdown of eleven days over the holidays. Maintenance procedures were in progress, which required the main central control rod to be manually withdrawn a few inches to reconnect it to its drive mechanism; at 9:01 p.m. this rod was suddenly withdrawn too far, causing SL-1 to go prompt critical instantly. In four milliseconds, the heat generated by the resulting enormous power surge caused water surrounding the core to begin to explosively vaporize. The water vapor caused a pressure wave to strike the top of the reactor vessel, causing water and steam to spray from the top of the vessel. This extreme form of water hammer propelled control rods, shield plugs, and the entire reactor vessel upwards. A later investigation concluded that the 26,000-pound (12,000 kg) vessel had jumped 9 feet 1 inch (2.77 m) and the upper control rod drive mechanisms had struck the ceiling of the reactor building prior to settling back into its original location.[7][10] The spray of water and steam knocked two operators onto the floor, killing one and severely injuring another. One of the shield plugs on top of the reactor vessel impaled the third man through his groin and exited his shoulder, pinning him to the ceiling.[7] The victims were Army Specialists John A. Byrnes (age 27) and Richard Leroy McKinley (age 22), and NavySeabeeConstruction ElectricianFirst Class (CE1) Richard C. Legg (age 26).[11][12] It was later established that Byrnes (the reactor operator) had lifted the rod and caused the excursion, Legg (the shift supervisor) was standing on top of the reactor vessel and was impaled and pinned to the ceiling, and McKinley, the trainee who stood nearby, was later found alive by rescuers.[7] All three men succumbed to injuries from physical trauma;[7] however, the radiation from the nuclear excursion would have given the men no chance of survival.

Several "kinetic" factors affect the rate at which the power (heat) produced in a nuclear reactor responds to changes in the position of a control rod. Other features of the design govern how rapidly heat is transferred from the reactor fuel to the coolant.

The nuclear chain reaction has a positive feedback component whenever a critical mass is created; specifically, excess neutrons are produced for every fission. Inside a nuclear reactor, these excess neutrons must be controlled as long as a critical mass exists. The most significant and effective control mechanism is the use of control rods to absorb the excess neutrons. Other controls include the size and shape of the reactor and the presence of neutron reflectors in and around a core. Changing the amount of absorption or reflection of neutrons will affect neutron flux, and therefore, the power of the reactor.

One kinetics factor is the tendency of most light-water-moderated reactor (LWR) designs to have negative moderator temperature and void coefficients of reactivity. A negative reactivity coefficient means that as the water moderator heats up, molecules move farther apart (water expands and eventually boils) and neutrons are less likely to be slowed by collisions to energies favorable for inducing fission in the fuel. Because of these negative feedback mechanisms, most LWRs will naturally tend to decrease their rate of fissioning in response to additional heat produced within the reactor core. If enough heat is produced that water boils inside the core, fissions in that vicinity will drastically decrease.

However, when power output from the nuclear reaction increases rapidly, it may take longer for the water to heat up and boil than it does for the voids to cause the nuclear reactions to decrease. In such an event, reactor power can grow rapidly without any negative feedback from the expansion or boiling of the water, even if it is in a channel just 1 cm away. Dramatic heating will occur to the nuclear fuel, leading to melting and vaporization of the metals within the core. Rapid expansion, increases in pressure, and failure of core components may lead to the destruction of the nuclear reactor, as was the case with SL-1. As the energy of expansion and heat travel from the nuclear fuel to the water and the vessel, it becomes likely that the nuclear reaction will shut down, either from the lack of sufficient moderator or from the fuel expanding beyond the realm of a critical mass. In the post-accident analysis of SL-1, scientists determined that the two shutdown mechanisms were almost equally matched (see below).

Another relevant kinetics factor is the contribution of what are called delayed neutrons to the chain reaction in the core. Most neutrons (the prompt neutrons) are produced nearly instantaneously via fission. But a few — approximately 0.7 percent in a U-235-fueled reactor operating at steady-state — are produced through the relatively slow radioactive decay of certain fission products. Delayed production of a fraction of the neutrons is what allows reactor power changes to be controllable on a time scale that is amenable to humans and machinery.[13] In the case of an ejected control assembly, it is possible for the reactor to become criticalon the prompt neutrons alone (i.e. prompt critical). When the reactor is prompt critical, the time to double the power is on the order of 10 microseconds. The duration necessary for temperature to follow the power level depends on the design of the reactor core. Typically, the coolant temperature lags behind the power by 3 to 5 seconds in a conventional LWR. In the SL-1 design, it was about 6 milliseconds before steam formation started.

SL-1 was constructed with a main central control rod that was capable of producing a very large excess reactivity if it were completely removed. The extra rod worth was in part due to the decision to load only 40 of the 59 fuel assemblies with nuclear fuel, thus making the prototype reactor core more active in the center. In normal operation, control rods are withdrawn only enough to cause sufficient reactivity for a sustained nuclear reaction and power generation. In this accident, however, the reactivity addition was sufficient to take the reactor prompt critical within a time estimated at 3.6 milliseconds. That was too fast for the heat from the fuel to get through the aluminum cladding and boil enough water to fully stop the power growth in all parts of the core via negative moderator temperature and void feedback.

Post-accident analysis concluded that the final control method (i.e., dissipation of the prompt critical state) occurred by means of catastrophic core disassembly: destructive melting, vaporization, and consequent conventional explosive expansion of the parts of the reactor core where the greatest amount of heat was being produced most quickly. It was estimated that this core heating and vaporization process happened in about 7.5 milliseconds, before enough steam had been formed to shut down the reaction, beating the steam shutdown by a few milliseconds. A key statistic makes it clear why the core literally blew apart: the reactor designed for a 3 MW power output operated momentarily at a peak of nearly 20 GW, a power density over 6,000 times higher than its safe operating limit.

There were no other people at the reactor site. The ending of the nuclear reaction was caused solely by the design of the reactor and the basic physics of heated water and core elements melting, separating the core elements and removing the moderator.

Heat sensors above the reactor set off an alarm at the central test site security facility at 9:01 p.m. MST, the time of the incident. False alarms had occurred in the morning and afternoon that same day. The first response crew, of six firemen (Ken Dearden Asst Chief, Mel Hess Lt., Bob Archer, Carl Johnson, Egon Lamprecht, Gerald Stuart, & Vern Conlon), arrived nine minutes later, expecting another false alarm.[14] and initially noticed nothing unusual, with only a little steam rising from the building, normal for the cold 6 °F (−14 °C) night. The control building appeared normal. The firefighters entered the reactor building and noticed a radiation warning light. Their radiation detectors jumped sharply to above their maximum range limit as they were climbing the stairs to SL-1's floor level. They were able to peer into the reactor room before withdrawing.[14]

At 9:17 p.m., a health physicist arrived. He and a fireman, both wearing air tanks and masks with positive pressure in the mask to force out any potential contaminants, approached the reactor building stairs. Their detectors read 25 röntgens per hour (R/hr) as they started up the stairs, and they withdrew.

Some minutes later, a health physics response team arrived with radiation meters capable of measuring gamma radiation up to 500 R/hr—and full-body protective clothing. One health physicist and two firefighters ascended the stairs and, from the top, could see damage in the reactor room. With the meter showing maximum scale readings, they withdrew rather than approach the reactor more closely and risk further exposure.

The stretcher rig. Army volunteers from a special Chemical Radiological Unit at Dugway Proving Ground practiced before a crane inserted the rig into the SL-1 reactor building to collect the body of the man impaled to the ceiling directly above the reactor vessel.

Around 10:30 p.m. MST, the supervisor for the contractor running the site (Combustion Engineering) and the chief health physicist arrived. They entered the reactor building and found two mutilated men soaked with water: one clearly dead (Byrnes), the other moving slightly (McKinley) and moaning. With one entry per person and a 1 minute limit, a team of 5 men with stretchers recovered the operator who was still breathing; he did not regain consciousness and died of his head injury at about 11 p.m. Even stripped, his body was so contaminated that it was emitting about 500 R/hr. The third man was discovered at about 10:38 p.m., impaled to the ceiling. With all potential survivors now recovered, safety of rescuers took precedence and work was slowed to protect them.

On the night of January 4, a team of six volunteers used a plan involving teams of two to recover the body of Byrnes. Radioactive gold 198Au from the man's gold watch buckle and copper 64Cu from a screw in a cigarette lighter subsequently proved that the reactor had indeed gone prompt critical. Up until the recovery of products of neutron exposure from the men's belongings, scientists had doubted that a nuclear excursion had occurred, thinking it inherently safe. These findings ruled out early speculations that a chemical explosion caused the accident.[10]

The third man was discovered last because he was pinned to the ceiling above the reactor by a shield plug and not easily recognizable.[7] On January 9, in relays of two at a time, a team of ten men, allowed no more than 65 seconds exposure each, used sharp hooks on the end of long poles to pull Legg's body free of the shield plug, dropping it onto a 5-by-20-foot (1.5 by 6.1 m) stretcher attached to a crane.[7]

The bodies of all three were buried in lead-lined caskets sealed with concrete and placed in metal vaults with a concrete cover. Some highly radioactive body parts were buried in the Idaho desert as radioactive waste. Army Specialist Richard Leroy McKinley is buried in section 31 of Arlington National Cemetery.

Some sources and eyewitness accounts confuse the names and disposition of each victim.[7] In Idaho Falls: The untold story of America's first nuclear accident,[15] the author indicates that Byrnes was the man found alive initially, Legg's body was recovered the night after the accident, and that McKinley was impaled by the control rod.

One of the required maintenance procedures called for the main control rod to be manually withdrawn approximately four inches (10 cm) in order to attach it to the automated control mechanism from which it had been disconnected. Post-incident calculations estimate that the main control rod was actually withdrawn approximately 26 inches (66 cm), causing the reactor to go prompt critical, which resulted in the steam explosion. The fuel, portions of the fuel plates, and water surrounding the fuel plates vaporized in the extreme heat. The expansion caused by this heating process caused water hammer as water was accelerated upwards toward the reactor vessel head, producing approximately 10,000 pounds per square inch (69,000 kPa) of pressure on the head of the reactor vessel when water struck the head at 160 feet per second (50 m/s).[16]

The water hammer not only caused extreme physical damage and distortion of the reactor vessel, it also caused the shield plugs of the vessel to be ejected, one of which impaled Legg. The most surprising and unforeseen evidences of the steam explosion and water hammer were the impressions made on the ceiling above the reactor vessel when it jumped over 9 feet (2.7 m) in the air before settling back into its prior location. The post-incident analysis also concluded that the reactor vessel was dry, since most of the water and steam had been either ejected immediately or evaporated due to the heat inside the reactor.

It was water hammer that caused the physical damage to the reactor, the deaths of personnel who stood atop and nearby, and the release of radioactive isotopes to the environment. One of the lessons learned from SL-1 was that there is an extreme water hammer hazard whenever a shutdown reactor is cooled to room temperature and there is an air gap between the top of the water and the reactor vessel head. One of the recommendations in the analysis of the incident was that shutdown reactors be filled to the top with water so that a power excursion could not induce such a powerful water hammer. Air is not dense enough to appreciably slow water, while water (being nearly incompressible) is able to distribute explosive forces and limit peak pressure. The extra water is also a very effective radiation shield for those who are directly above the vessel. Written Procedures at SL-1 had included a directive to pump down the level of water in the reactor prior to the maintenance procedure that destroyed it.

The most common theories proposed for the withdrawal of the rod so far are (1) sabotage or suicide by one of the operators, (2) a suicide-murder involving an affair with the wife of one of the other operators, (3) inadvertent withdrawal of the main control rod, or (4) an intentional attempt to "exercise" the rod (to make it travel more smoothly within its sheath).[17][18] The maintenance logs do not address what the technicians were attempting to do, and thus the actual cause of the incident will never be known. The investigation took almost two years to complete.

Investigators analyzed the flux wires installed during the maintenance to determine the power output level. They also examined scratches on the central control rod. Using this data, they concluded that the central rod had been withdrawn 26.25 inches (66.7 cm).[10] The reactor would have been critical at 23 inches (58.4 cm), and it took approximately 100 ms for the rod to travel the final 3.25 inches (8.3 cm). Once this was calculated, experiments were conducted with an identically weighted mock control rod to determine whether it was possible or feasible for one or two men to have performed this. Experiments included a simulation of the possibility that the 84-pound (38 kg) rod was stuck and one man freed it himself, reproducing the scenario that investigators considered the best explanation: Byrnes broke the control rod loose and withdrew it accidentally, killing all three men.[7]

The mechanical and material evidence, combined with the nuclear and chemical evidence, forced them to believe that the central control rod had been withdrawn very rapidly. […] The scientists questioned the [former operators of SL-1]: “Did you know that the reactor would go critical if the central control rod were removed?” Answer: “Of course! We often talked about what we would do if we were at a radar station and the Russians came. We’d yank it out.”

The remains of the SL-1 reactor are now buried near the original site at 43°31'02.9"N 112°49'22.2"W.[19]

The incident caused this design to be abandoned and future reactors to be designed so that a single control rod removal would not have the ability to produce the very large excess reactivity which was possible with this design. Today this is known as the "one stuck rod" criterion and requires complete shutdown capability even with the most reactive rod stuck in the fully withdrawn position. The reduced excess reactivity limits the possible size and speed of the power surge. It should be pointed out that the "one stuck rod" criterion did not originate as a result of the SL-1 incident. It was, in fact, a hard and fast design criterion long before the SL-1, from the beginning of the Naval Reactors program, under the leadership of Admiral Hyman Rickover. This design criteria started with the USS Nautilus, and continued throughout subsequent submarine and surface ship designs, and with the Shippingport civilian nuclear plant. It continues to be a rigid requirement for all US reactor designs to this day.

The incident also showed that in a genuine, extreme accident, both the melting of the core and the water to steam conversion would shut down the nuclear reaction. This demonstrates in a real accident one aspect of inherent safety of the water-moderated design against the possibility of a nuclear explosion.

A nuclear explosion requires sufficient force to hold the reacting nuclear components together for a short but necessary time. This is achieved in a nuclear fission weapon by surrounding the core with a carefully engineered tamper (typically U-238), and a shaped explosive charge. This, along with other subsystems of the weapon keep the supercritical mass together long enough for sufficient generations of the fission reaction to produce the desired yield. Lacking these restraints to hold the vaporized core components together, the components of a reactor fly apart, as in this incident. The reaction ends, resulting in a steam explosion and a badly damaged reactor core, but not the type of explosion as would be achieved with a nuclear weapon.

Even without an engineered containment building like those used today, the SL-1 reactor building contained most of the radioactivity, though iodine-131 levels on plants during several days of monitoring reached fifty times background levels downwind.

Radiation exposure limits prior to the incident were 100 röntgens to save a life and 25 to save valuable property. During the response to the incident, 22 people received doses of 3 to 27 Röntgens full-body exposure.[20] Removal of radioactive waste and disposal of the three bodies eventually exposed 790 people to harmful levels of radiation.[21] In March 1962, the Atomic Energy Commission awarded certificates of heroism to 32 participants in the response.

The documentation and procedures required for operating nuclear reactors expanded substantially, becoming far more formal as procedures which had previously taken two pages expanded to hundreds. Radiation meters were changed to allow higher ranges for emergency response activities.

After a pause for evaluation of procedures, the Army continued its use of reactors, operating the Mobile Low-Power Reactor (ML-1), which started full power operation on February 28, 1963, becoming the smallest nuclear power plant on record to do so. This design was eventually abandoned after corrosion problems. While the tests had shown that nuclear power was likely to have lower total costs, the financial pressures of the Vietnam War caused the Army to favor lower initial costs and it stopped the development of its reactor program in 1965, although the existing reactors continued operating (MH-1A until 1977).

The site was cleaned in 1961 to 1962, removing the bulk of the contaminated debris and burying it. The massive cleanup operation included the dismantling and disposal of the reactor and building. A burial ground was constructed approximately 1,600 feet (500 m) northeast of the original site of the reactor. This was done to minimize radiation exposure to the public and site workers that would have resulted from transport of contaminated debris from SL-1 to the Radioactive-Waste Management Complex over 16 miles (26 km) of public highway. Original cleanup of the site took about 18 months. The entire reactor building, contaminated materials from nearby buildings, and soil and gravel contaminated during cleanup operations were disposed of in the burial ground. The majority of buried materials consist of soils and gravel.[22][23]

Recovered portions of the reactor core, including the fuel and all other parts of the reactor that were important to the incident investigation, were taken to the INEL's Test Area North for study. After the incident investigation was complete, the reactor fuel was sent to the Idaho Chemical Processing Plant for reprocessing. The reactor core minus the fuel, along with the other components sent to Test Area North for study, was eventually disposed of at the Radioactive Waste Management Complex.[22]

The SL-1 burial ground consists of three excavations, in which a total volume of 99,000 cubic feet (2800 m3) of contaminated material was deposited. The excavations were dug as close to basalt as the equipment used would allow and ranges from 8 to 14 feet (2.4 to 4.3 m) in depth. At least 2 feet (0.6 m) of clean backfill was placed over each excavation. Shallow mounds of soil over the excavations were added at the completion of cleanup activities in September 1962. The site and burial mound are collectively known as United States Environmental Protection AgencySuperfund Operable Unit 5-05.[22][24]

Numerous radiation surveys and cleanup of the surface of the burial ground and surrounding area have been performed in the years since the SL-1 incident. Aerial surveys were performed by EG&G Las Vegas in 1974, 1982, 1990, and 1993. The Radiological and Environmental Sciences Laboratory conducted gamma radiation surveys every 3 to 4 years between 1973 and 1987 and every year between 1987 and 1994. Particle-picking at the site was performed in 1985 and 1993. Results from the surveys indicated that cesium-137 and its progeny (decay product) are the primary surface-soil contaminants. During a survey of surface soil in June 1994, "hot spots," areas of higher radioactivity, were found within the burial ground with activities ranging from 0.1 to 50 milliroentgen (mR)/hour. On November 17, 1994, the highest radiation reading measured at 2.5 feet (0.75 m) above the surface at the SL-1 burial ground was 0.5 mR/hour; local background radiation was 0.2 mR/hour. A 1995 assessment by the EPA recommended that a cap be placed over the burial mounds. The primary remedy for SL-1 was to be containment by capping with an engineered barrier constructed primarily of native materials.[22] This remedial action was completed in 2000 and first reviewed by the EPA in 2003.[24]

The U.S. Government produced a film about the incident for internal use in the 1960s. The video was subsequently released and can be viewed at The Internet Archive[25] and YouTube. SL-1 is the title of a 1983 movie, written and directed by Diane Orr and C. Larry Roberts, about the nuclear reactor explosion.[21] Interviews with scientists, archival film, and contemporary footage, as well as slow-motion sequences, are used in the film.[26][27] The events of the incident are also the subject of two books[clarification needed], one published in 2003, Idaho Falls: The untold story of America's first nuclear accident.[15]

In 1975 the anti-nuclear book We Almost Lost Detroit, by John G. Fuller was published, referring at one point to the Idaho Falls incident. Prompt Critical is the title of a 2012 short film, viewable on Youtube.com, written and directed by James Lawrence Sicard, dramatizing the events surrounding the SL-1 incident.

This image of the SL-1 core served as a sober reminder of the damage that a nuclear meltdown can cause.

Another author, Todd Tucker, studied the incident and published a book detailing the historical aspects of nuclear reactor programs of the U.S. military branches. Tucker used the Freedom of Information Act to obtain reports, including autopsies of the victims, writing in detail how each person died and how parts of their bodies were severed, analyzed, and buried as radioactive waste.[7] The autopsies were performed by the same pathologist known for his work following the Cecil Kelley criticality accident. Tucker explains the reasoning behind the autopsies and the severing of victims' body parts, one of which gave off 1,500 R/hour on contact. Because the SL-1 accident killed all three of the military operators on site, Tucker calls it "the deadliest nuclear reactor incident in U.S. history."[28]

SL-1 Accident Briefing Film Report, Atomic Energy Commission video, 1961. This video explains and diagrams the 9 foot "jump" of the reactor vessel and shows mock-up video of people attempting to reproduce the withdrawal of the central control rod.